$CI_COMMIT_REF_NAME can be a branch name or a tag, depending on the
pipeline, but our checkout script only deals with the former at the
moment.
Address this by rather than looking for a remote branch name, just
try to fetch the ref like we do for merge request pipelines.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1630>
We are currently not very good at communicating what's going in,
in particular for non-merge request pipelines where the output
is usually just "Using origin/master instead" (instead of what?).
Improve this by consistently communicate what we are looking for,
whether we found it, and what we end up doing.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1630>
For stable branches, we currently only check out the correct shell
branch for merge requests. For the regular pipeline, our code to
determine the current mutter branch fails because CI runs on a
temporary "pipeline/12345" branch that doesn't exist for gnome-shell.
Switching to the correct gitlab environment variable fixes that.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/811
